The breast is suggested to be most sensitive to hormonal and dietary exposures during fetal life, puberty and pregnancy; i.e., when it is growing extensively. It has been shown that high birthweight and excessive pregnancy weight gain increase sporadic breast cancer risk. High body weight during childhood paradoxically reduces sporadic breast cancer risk. We hypothesize that familial breast cancer risk is also affected by birthweight, weight during childhood and early puberty, and pregnancy weight gain. The 525 breast cancer families included in this study have been identified and collected as a part of ongoing hereditary breast cancer research programs at the Helsinki University Central Hospital, Tampere University Hospital and Oulu University Hospital. All patients participating in the studies of inherited breast cancer have signed an informed consent giving also permission for a later contact concerning this research. Questionnaires will be mailed to 836 cases and information on birth weight, body size during childhood and pregnancy weight gain as well as reproductive history and lifestyle including current diet will be obtained. Cases will also be requested to forward a questionnaire to their close female family members (sisters or cousins) who have not developed breast cancer. These women will serve as controls. Another control group consists of 1,672 women diagnosed with sporadic breast cancer; i.e., they have no family history of breast cancer. Data for BRCA1- and BRCA2- mutation carriers, and member of all other families who are not carriers of either of these mutations will be analysed separately. The results will determine whether body weight at specific developmental periods alters penetrance of breast cancer in women at high familial risk. If any associations are found, novel dietary-based intervention strategies may be developed and implemented to prevent women at high familial risk from developing breast cancer.